In my opinion, in the epi "Through the Looking Glass" everything changed, meaning our Losties' personalities. I've posed this before, but in the book "Through the Looking Glass" everything is exactly the opposite of what is reality. How does this relate to LOST? The writers are very clever in naming their episodes to give us clues as to the story right under our noses. I believe in that episode alone, we saw so many changes to our characters' personalities, Sun included...
but was she always so sweet? What her story has told us through 4 seasons is that she's capable of a great many things, not all of them sweet==quite the contrary. My point is that she's always been capable of dark things, including when she was a child. The apple doesn't fall far from the tree, my Lostie friend 
see you in another life, brothah
cheers
*Des8
Well, if you consider lying a dark side trait then we all are evil.

Not debating this as the point is simply this, Sun has had strong parents who have made her dress, date, and act certain ways. Part of the culture? Part of the financial status which result in her wanting to rebel.
She found the man she loved with Jin on this island again, it gave them a child she wasn't suppose to have. Then all that was taken away. Her vengance/revenge or whatever you want to call it is what was written. They do this to make the show move forward.
Yes, she has targets, Jack, her father and it appears Widmore maybe the other.
Whatever the writers write is what Sun will be.
PS, killing Colleen was killing in self defense....